For an image to be perceived clearly, the optics of the eye should result in an image that is focussed on the retina. Myopia, commonly known as short-sightedness, is an optical disorder of the eye wherein on-axis images are focussed in front of the fovea of the retina. Hyperopia, commonly known as long-sightedness, is an optical disorder of the eye wherein on-axis images are focussed behind the fovea of the retina. The focussing of images in front of or behind the fovea of the retina creates a lower order aberration of defocus. Another lower order aberration is astigmatism. An eye may also have higher order optical aberrations, including for example spherical aberration, coma and/or trefoil. Many people experiencing natural refractive error are progressing (the refractive error is increasing over time). Progression is particularly widespread in people with myopia. Schematic representations of eyes exhibiting myopia or hyperopia and astigmatism are shown in FIGS. 1A-C respectively. In a myopic eye 100, the parallel incoming beam of light 102 passes the refractive elements of the eye, namely, the cornea 104 and crystalline lens 106, to a focal point 108 short of the retina 110. The image on the retina 110 is therefore blurred. In a hyperopic eye 120, the parallel incoming beam of light 122 passes the refractive elements of the eye, namely, the cornea 124 and crystalline lens 126, to a focal point 128 beyond the retina 130, again rendering the image on the retina 130 blurred. In an astigmatic eye 140, the parallel incoming beam of light 142 passes the refractive elements of the eye, namely, cornea 144 and crystalline lens 146, and results in two foci, namely tangential 148 and sagital 158 foci. In the example of astigmatism shown in FIG. 1C, the tangential focus 148 is in front the retina 160 while the sagital focus 158 is behind the retina 160. The image on the retina in the astigmatic case is referred to as circle of least confusion 160.
At birth human eyes are hyperopic, i.e. the axial length of the eyeball is too short for its optical power. With age, from infancy to adulthood, the eyeball continues to grow until its refractive state stabilizes. Elongation of the eye in a growing human may be controlled by a feedback mechanism, known as the emmetropisation process, so that the position of focus relative to the retina plays a role in controlling the extent of eye growth. Deviation from this process would potentially result in refractive disorders like myopia, hyperopia and/or astigmatism. While there is ongoing research into the cause of deviation of emmetropisation from stabilising at emmetropia, one theory is that optical feedback can provide a part in controlling eye growth. For example, FIG. 2 shows cases that would, under a feedback mechanism theory of the emmetropisation process, alter the emmetropisation process. In FIG. 2A, the parallel incoming beam of light 202 passes through a negative refractive element 203 and the refractive elements of the eye (the cornea 204 and crystalline lens 206), to form an image at focus point 208, overshooting the retina 210. The resulting image blur on the retina, called hyperopic defocus, is an example of defocus that may encourage eye growth under this feedback mechanism. In contrast, as seen in FIG. 2B, the parallel incoming beam of light 252 passes through a positive refractive element 253, the refractive elements of the eye (cornea 254 and crystalline lens 256) to form an image at focus point 258 in front of the retina 260. The resulting image blur, called myopic defocus, on this retina is considered to be an example of defocus induced at the retina that would not encourage eye growth. Therefore, it has been proposed that progression of myopic refractive error can be controlled by positioning of the focus in front of the retina. For an astigmatic system, the spherical equivalent, i.e. the mid-point between the tangential and sagital foci, may be positioned in front of the retina. These proposals have not however provided a full explanation or solution, particularly in the case of progressing myopia.
A number of optical device designs and refractive surgery methods have been proposed to control the growth of the eye during emmetropisation. Many are generally based on refinements to the idea summarised above that foveal imagery provides a stimulus that controls the growth of the eye. In humans, the eye grows longer during emmetropisation and can not grow shorter. Accordingly, during emmetropisation an eye may grow longer to correct for hyperopia, but it can not grow shorter to correct for myopia. Many proposals have been made for addressing myopia progression, some of which are summarised below.
U.S. Pat. No. 6,752,499 (Aller) proposes the use of bifocal contact lenses for myopic participants who exhibit near-point esophoria, for providing a stimulus for reducing/controlling myopia progression. U.S. Pat. No. 7,025,460 (Smith et al) proposes the use of corrective eye lenses that shift the focal plane in front of the peripheral retina. U.S. Pat. No. 7,506,983 (To et al) proposes a method of treating myopia progression in human eyes by producing a secondary myopic image by use of Fresnel optics, while correcting the myopia of the candidate via the refractive portion of the lens. U.S. Pat. No. 7,997,725 (Phillips) proposes a method of simultaneous vision, wherein one part of the correcting lens corrects for pre-existing myopia while another part has less negative power than the focal power of the lens to be able to produce simultaneous myopic defocus and thereby aid in the retardation of myopia progression. U.S. Pat. No. 6,045,578 (Collins and Wildsoet) proposes the addition of positive spherical aberration at the fovea to provide a stimulus that will reduce and/or control myopia progression. U.S. Pat. No. 7,401,922 (Legerton et al) proposes a method and system of treating myopia progression in myopic patients by inducing certain aberration profiles that have positive spherical aberration to produce a wavefront disposed in front of the retina. U.S. Pat. No. 7,803,153, B2 (Thorn et al) proposes a method of preventing myopia progression through identification and correction of all optical aberrations, including higher order aberrations.
In addition to proposed optical strategies to counter the development of refractive error and its progression, in particular myopia, there has also been interest in strategies that involve non-optical intervention like pharmacological substances, such as atropine or pirenzipine.
Another condition of the eye is presbyopia, in which the eye's ability to accommodate is reduced or the eye has lost its ability to accommodate. Presbyopia may be experienced in combination with myopia, hyperopia, astigmatism and higher order aberrations. Many different methods, devices and lenses to address presbyopia have been proposed, including in the form of bifocal, multifocal or progressive addition lenses/devices, which simultaneously provide two or more foci to the eye. Three common types of lenses used for presbyopia are centre-near, centre-distance aspheric multifocals and concentric (ring-type) bifocals alternating between distance and near powers.
In addition, on occasion it is necessary to remove the crystalline lens of an eye, for example if the person is suffering from cataracts. The removed natural crystalline lens may be replaced by an intraocular lens. Accommodating intraocular lenses allow the eye to control the refractive power of the lens, for example through haptics extending from the lens to the ciliary body.